A smoke detector is a device that senses smoke, typically as an indicator of fire. Commercial security devices issue a signal to a fire alarm control panel as part of a fire alarm system, while household smoke detectors generally issue a local audible or visual alarm from the detector itself. Many smoke detectors apply light scattering for smoke detection. This has the advantages of fast response and low signal drift, as well as low cost and long useful life. The reliability of such a smoke detector may be impaired by the presence of aerosols like water vapor, cigarette smoke, dust or various fumes, which may cause false alarm.
A light detector that is used in a smoke detector may be mounted in a smoke chamber, which shields ambient light but allows smoke to enter and reach the light detector. A smoke chamber enlarges the device and may unduly delay the detection of smoke, if the passage of the smoke to the light detector is inhibited by parts of the smoke chamber or by the shape or arrangement of the inlet. If the smoke detector is exposed to the environment, dust or dew may adhere to the smoke chamber in the course of time and may cause false alarms.
U.S. Pat. No. 6,218,950 B1 discloses a fire detector measuring light signals at two different scattering angles. Two optical paths for the scattered light are arranged in such a way that one of the paths responds predominantly to light aerosols whereas the other path responds predominantly to dark aerosols. An alarm threshold is determined depending on the brightness of the aerosol. Fraudulent measurement values arising from water vapor or the like are stored in a memory for further reference to avoid false alarms.
U.S. Pat. No. 6,788,197 B1 discloses a fire alarm device having an electronic evaluator, an optical module, a temperature sensor and at least one combustion gas sensor. A light source, which can be an LED, emits non-polarized light. A polarizer with a rotatable polarization plane, which may comprise a liquid crystal, is arranged between the light source and an optical receiver. Thus scattering of the emitted light can be measured in two orthogonal polarization planes.
U.S. Pat. No. 7,746,239 B2 discloses a light scattering type smoke detector including a sensor body, a light-emitter for emitting light toward an open smoke-sensing space and outputting a signal according to the amount of scattered light received, and a fire judging unit. Two light sources with polarization filters emit at different angles with different polarization states, and forward and backwards scattered light is collected by a light receiving element.
CN 200963473 Y discloses a photoelectric smoke fire detecting device based on depolarization. A light beam emitted from a light source is injected to a smoke granule via a polarizer, taking the plane passed by the incident light beam as a scattering base plane. The upper and lower sides of a receiving plane, which is vertical to the scattering base plane, are provided with two identical photoelectric receivers, which are placed close to and symmetrical to the scattering base plane with and without a polarizer which has a polarization axis vertical to the scattering plane.
In the publication of Stefano di Stasio, “Experiments on depolarized optical scattering to sense in situ the onset of early agglomeration between nano-size soot particles”, Journal of Quantitative Spectroscopy and Radiative Transfer vol. 73 (2002), pages 423-432, soot aerosol particles generated in hydrocarbon flames are investigated by laser light scattering techniques. The contribution of co- and cross-polarized scattered light for vertical and horizontal linear polarization states are measured at variable polar angles. The vertical depolarization ratio measured against the scattering angle was found to be very low and flat in the case of smaller chain-like aggregates, whereas in the case of larger branched-chain aggregates, it exhibits a maximum at about 90°. The measured ratio between the two depolarized contributions for each of the vertical and horizontal polarization states of the incident light is found to be suitable to establish the onset of the early aggregation mechanism between nano-size soot particles formed in a flame.
When a light ray passes a planar boundary between two isotropic dielectric media, from a first medium having a refractive index n1 to a second medium having a different refractive index n2, the incident ray is split into a reflected ray and a refracted ray. The incident, reflected and refracted rays propagate in the same plane, which is designated as plane of incidence.
The angle of incidence θ1 is enclosed by the incident ray and a straight line normal to the boundary, the straight line intersecting with the incident ray. The angle of refraction θ2 is enclosed by the refracted ray and a straight line normal to the boundary, the straight line intersecting with the refracted ray. The indices n1 and n2 and the angles θ1 and θ2 are connected by Snell's law: n1·sin θ1=n2·sin θ2.
The refracted ray vanishes if n2<n1 and θ1 is so large that the equation n1·sin θ1=n2·sin θ2 cannot be satisfied. This condition is called total internal reflection. The limit occurs for θ2=90°, in which case the angle of incidence is the critical angle θ1=arcsin(n2/n1).
A component of the polarization that is normal to the plane of incidence is conventionally designated as s-polarization, and a component of the polarization that is parallel to the plane of incidence is designated as p-polarization.
The reflectance is the fraction of the incident power that is reflected from the interface between the dielectric media. From the Fresnel equations for non-absorbing, non-magnetic materials, the value of the reflectance for p-polarization is [(n1·cos θ2−n2·cos θ1)/(n1·cos θ2+n2·cos θ1)]2.
The value of the reflectance is zero if n1·cos θ2=n2·cos θ1, which is equivalent to n1·sin(90°−θ2)=n2·sin(90°−θ1). This condition is met if θ1+θ2=90°, according to Snell's law, and occurs for a special angle of incidence θB, which is named Brewster's angle after the Scottish physicist David Brewster.
The equation n1·sin θB=n2·sin θ2=n2·sin(90°−θB)=n2·cos θB, which is valid in the case θB+θ2=90°, yields the relation θB=arctan (n2/n1). For a boundary between a clear molding compound and ambient air, Brewster's angle may be typically θB=33.85°.